1. Field of the Invention
The present invention relates to a wireless communication apparatus and a wireless communication system for performing wireless communication. More particularly, the present invention relates to a wireless communication apparatus and a wireless communication system that employ a multiple-carrier transmission method such as the OFDM (orthogonal frequency division multiplexing) method for transmitting AV streams and data on a wireless basis.
2. Description of the Prior Art
In recent years, as methods for transmitting data and AV streams on a wireless basis, among others, those complying with IEEE802.11b, using the 2.4 GHz band, and IEEE802.11a, using the 5.2 GHz band, have been becoming increasingly popular. Radio waves in the 2.4 GHz band tend to propagate less rectilinearly than those in the 5.2 GHz band, and this gives the former the advantage of readily propagating around the corners of a shielding object and the advantage of offering longer wireless transmission distances. However, the 2.4 GHz frequency band is used comparatively freely, and is shared by other communication methods such as Bluetooth and by the radio waves emitted from microwave ovens. Thus, in the 2.4 GHz frequency band, interference with such other microwaves tends to lower the effective throughput of communication.
Accordingly, in a case where the method complying with IEEE802.11b is used for data transmission such as file transfer in a wireless LAN (local area network), it is not absolutely necessary to guarantee any specific QoS (quality of service), i.e., to give a guarantee that a particular amount of data is transmitted within a particular length of time. Thus, interference with unrelated radio waves does not cause any problem. However, in a case where high-quality images and sounds are transmitted on a wireless basis, it is necessary to guarantee a certain QoS, and therefore interference with unrelated radio waves causes an unacceptable lowering of the effective throughput.
By contrast, the method complying with IEEE802.11a, which uses the 5.2 GHz band, adopts the OFDM method, and, by using a plurality of subcarriers spread over a wide frequency band, achieves a high physical transmission rate and a high effective throughput. Thus, this method creates a transmission state that offers efficiency high enough to guarantee a desired QoS. However, radio waves in the 5.2 GHz band tend to propagate more rectilinearly than those in the 2.4 GHz band, and this gives the former the disadvantage of less readily propagating around the corners of a shielding object and the disadvantage of offering shorter wireless transmission distances. Thus, in wireless transmission performed over a wide frequency band, when frequency-selective fading occurs due to factors in the indoor or outdoor transmission environment, reception condition may become poor with particular subcarriers only, resulting in the missing of the data transmitted by those subcarriers.
To prevent such missing of the data carried by a particular subcarrier, there has been conventionally proposed a diversity receiver wherein reception is achieved by the use of a plurality of antennas and, for each subcarrier, the signals received via the different antennas are corrected and then synthesized together, or a diversity receiver wherein, for each subcarrier, the signal strengths or the like of the signals received via a plurality of antennas are compared with one another so that a signal being received in good reception condition is selected and demodulated (see Japanese Patent Application Laid-Open No. 2000-36801). There has also been conventionally proposed a diversity receiver wherein a plurality of antennas are used and, for each subcarrier, the carrier levels of the signals received via the different antennas are checked so that a signal being received in good reception condition is selected (see Japanese Patent Application Laid-Open No. 2000-174726).
Thus, for example, in a case where the frequency band used is the 5.2 GHz band as with the method complying with IEEE802.11a, by using the diversity receiver proposed in one of the patent publications mentioned above, it is possible to receive OFDM signals via a plurality of antennas. Here, an attempt is made to prevent the deterioration of subcarriers by exploiting the differences in characteristics of the frequency-selective fading occurring in the individual OFDM signals received by the different antennas.
However, the diversity receivers proposed in the Japanese Patent Applications Laid-Open Nos. 2000-36801 and 2000-174726 mentioned above use only one frequency band, and therefore, when the plurality of antennas are arranged close to one another spatially, the signals received by the different antennas may show similar frequency-selective fading characteristics. In that case, the subcarriers of an identical frequency obtained from the OFDM signals received via those antennas exhibit similar degrees of deterioration. Thus, the result of synthesis of those subcarriers, or the result of comparison among and selection from them, may be less accurate than the results for other subcarriers.
In particular, in a case where OFDM signals in the 5.2 GHz band are received, when the plurality of antennas are arranged close to one another spatially, all the relevant radio waves may fail to propagate around in similar manners with respect to the different antennas, or may fail to reach the antennas due to the transmission loss that the radio waves in that frequency band inevitably suffer along the spatial transmission paths. This results in poor reception condition via all the antennas.